Recently, several methods are used in security techniques to detect persons or animals present in closed and open spaces. It concerns mainly devices utilizing the principles of infrared radiation, microwave radiation, ultrasound, X-ray radiation, changes induced by vibrations, changes of images recorded by camera systems, measurement of variation of capacitance. Also changes in optical fibres and in high-frequency electromagnetic fields are used to detect persons and animals.
When evaluating changes in high-frequency electromagnetic fields, in general a system is used, where the evaluating unit consists of two oscillators, of which one has a precisely set frequency and serves as a reference. The second oscillator is detuned by stray capacitance as a consequence of approaching persons, animals or things. The detuning is then measured and evaluated. Using this system, we are unable to distinguish, if the disturbance is caused by a person, animal or a thing. Moreover, stability of these devices is influenced by many different factors, like humidity, static discharges, movements of trees or other things in the electromagnetic field, unstable geomagnetic field of the Earth and the like, in consequence of which this method cannot be used in security technology.
With the above method, it is impossible to distinguish, if the electromagnetic field disturbance is caused by a living organism or another physical factor.